drm/i915: Remove a bad BUG_ON in the fence management code.

This could be triggered by a gtt mapping fault on 965 that decides to
remove the fence from another object that happens to be active currently.
Since the other object doesn't rely on the fence reg for its execution, we
don't wait for it to finish.  We'll soon be not waiting on 915 most of the
time as well, so just drop the BUG_ON.

Signed-off-by: Eric Anholt <eric@anholt.net>